Role of calcium influx in the regulation of sugar transport in resting left atrial muscle.

The role of external Ca2+ in the regulation of sugar transport in isolated resting atria of rats and guinea pigs was studied by measuring the tissue/medium distribution of 14C-labelled 3-methylglucose and of 45Ca. Omission of Ca2+ from the medium strongly antagonized the stimulation of sugar transport by insulin, hyperosmolarity (100 mM mannitol) or Na+-pump inhibition (K+-free medium). Basal sugar transport was not affected by Ca2+ omission but was increased when 0.5 mM EGTA was also added. The Ca2+ antagonist drug D-600 decreased 45Ca influx and also inhibited sugar-transport stimulation by the above 3 treatments in the rat, while in the guinea pig it antagonized only insulin-stimulated transport. The stimulation of sugar transport by a high adrenaline concentration and the inhibition by a low concentration were both antagonized by Ca2+ omission or D-600. The results illustrate the important role of Ca2+ influx in the control of sugar transport by hormonal and other modulators and are consistent with the hypothesis that cytoplasmic Ca2+ regulates glucose transport in muscle.